The present disclosure relates to lighting fixtures, and more particularly to outdoor lighting fixtures for distributing patterns of light on the ground. These lighting fixtures can be used for area lighting, including roadway, parking lot, walkway, bicycle path, or other similar applications.
In general, roadway lighting fixtures consist of a lamp or other light source, a lens, and a reflector for refracting and/or reflecting light from the light source. The reflector, lens and any shielding typically define the light distribution pattern.
Highway and roadway lighting have historically used incandescent and more recently high intensity discharge (HID) lamps that can provide adequate amounts of light, but which have several drawbacks, including frequent lamp failures and poorly distributed lighting of the roadway surface. Incandescent and HID lamps are omni-directional sources and have relatively poor control of the light which results in lower utilization. Uncontrolled light can be wasted in lighting areas around the roadway (and potentially, sidewalk) that do not require light, and contributes to trespass light and light pollution which can interfere with the preservation of the nighttime environment.
As advances in the quality and energy efficiency of lighting sources such as light emitting diodes (LEDs) have improved, their production costs have gone down. As a result, LEDs, are being more commonly used in outdoor lighting applications. Initial efforts to incorporate LEDs into lighting fixtures have involved retrofitting LEDs into conventional luminaries or onto or into the shape of conventional lighting luminaires.
LEDs provide an effective means to achieve targeted illumination. However, careful design of the luminaire package is required. Light energy spreads over an area as a function of distance. The illumination of a remote area therefore varies inversely as the square of the distance from the light source. Additionally, since light fixtures direct light to a relatively large target area, the light source is many times smaller than the area to be lighted. Accordingly, the luminaire produced by each fixture must be relatively intense to cover a substantial area.
FIGS. 1A to 1G show types of roadway illumination patterns. These are designed to provide effective illumination of various conditions. Moreover, a roadway luminaire should be capable of illuminating one or several lane roadways, should accommodate a variety of pole spacing's, and may be required to provide backwards illumination of a sidewalk, to name just a few exemplary requirements. The Illuminating Engineering Society of North America (IESNA) is an accepted technical authority on illumination and puts out specifications for five primary types of roadway illumination.
The classification type is defined by the half maximum iso-candella line in relation to street side transverse (across the road) mounting heights. This is independent of the longitudinal (along the road) capability which is defined by the relationship of the projected maximum candela to longitudinal mounting heights. The type classification represents the amount of forward throw of the distribution and can be generally equated to the number of lanes or distance of coverage.
Type I illumination, FIG. 1A, is a direct illumination in two directions along the direction of the roadway wherein the lamp post can be median mounted between opposite flows of traffic and/or in a straight directional pattern at a cross section as shown in FIG. 1B. FIG. 1C shows an omni-directional lighting pattern across the entire intersection. FIG. 1D shows a lighting fixture which directs light at an angle (asymmetrically) to normal in either two directions, or in four directions as shown in FIG. 1E, Type II illumination. Type III illumination in FIG. 1F shows a greater angle, or illumination from normal as compared to Type II (FIG. 1D). Type IV illumination (FIG. 1G) has an even wider angle of illumination from normal. As described above, these illumination patterns are desired to effect lighting of various application conditions.
There are additional problems presented to the lighting designer. First of all, to maintain a given light level at a distant target area, the light source must produce a high level of light intensity. This can contribute to glare problems for those viewing the fixtures. Spill and glare are inefficient use of the light and are frequently objectionable. Spill light primarily wastes energy and should be minimized although some controlled spill light is necessary to provide a gradient and light the roadway peripherals. Spill results in wide-scale lighting of areas, which makes the actual roadway less distinct from surrounding areas. Additionally, lack of control also translates, in many applications, into the utilization of more light poles and lighting fixtures, which is expensive and consumes substantial resources.
Having a light engine which is adaptable to provide a wide array of light distribution patterns allows precise control of light. One advantage of the present disclosure is that by providing an adaptable modular lighting fixture, it is feasible to readily select fixture modules having suitable light distribution and orientation to properly light almost any area.